Continuing a regular theme that ‘we cannot manage what we do not know’, regular correspondent Daryl Sykes looks at reliable fisheries management decisions and ecosystem based management through the…
It is considered best practice to ensure that fisheries management decisions are informed by good science, through the use of data and research, to understanding the biology, population dynamics, and habitat needs of the species being managed—as well as the impacts of various management strategies.
Good science includes the use of multiple sources of information, transparency and openness in the process, and a system of continuous improvement. Information can then be used to make informed decisions about how to sustainably manage the fishery and protect the fish populations and habitats.
Essential data required for stock assessment includes information on the biology, life history, and population dynamics of the species; data on fishing effort, including gear types and locations; and data on the size, age, and sex composition of the catch.
Other data important for a comprehensive stock assessment may include: information on the habitats and ecosystems that support the stock; environmental factors such as water temperature, ocean currents, and weather patterns; the economic and social aspects of the fishery, such as the value of the catch and the number of people employed in the fishery.
If catch and effort data are incomplete, it is more difficult to estimate the population size and health of a fish stock, making the stock assessment less reliable.
Correct input for assessment models
Catch data are essential to estimate the population size and fishing mortality rate, which are key parameters for stock assessment models. Incomplete or inaccurate catch data can lead to biased estimates of population size and fishing mortality rate, which in turn can lead to poor management decisions.
Catch effort data is also important, as it allows for the estimation of fishing mortality rate, which is a key parameter for stock assessment models. Incomplete or inaccurate effort data can lead to biased estimates of fishing mortality rate, which in turn can lead to poor management decisions.
Indicators of abundance
Several indicators of stock abundance can be derived from fisheries data, including:
Catch per unit effort (CPUE): This measures the amount of fish caught per unit of effort, such as per hour of fishing or per unit of fishing gear. An increase in CPUE can indicate an increase in stock abundance.
Length frequency: This measures the distribution of fish lengths in a sample population. Changes in the length frequency can indicate changes in the age structure of the population, which can be used to infer changes in stock abundance.
Age structure: This measures the proportion of fish of different ages in a population. Changes in the age structure can indicate changes in the recruitment (the number of new fish entering the population) and mortality rates, which can be used to infer changes in stock abundance.
Biomass estimates: This measures the total weight of fish in a population. Biomass estimates can be calculated using catch data, acoustic surveys, or other methods. An increase in biomass can indicate an increase in stock abundance.
Stock assessment: Stock assessment is a more comprehensive approach that combines all the above data and uses statistical models to estimate the size and health of fish populations, which can be used to infer changes in stock abundance.
Ecosystem effects can be factored into a fisheries stock assessment in several ways by considering the interactions between the target fish population and other species in the ecosystem.
Trophic interactions: By considering the food web relationships, the assessment can take into account the effects of fishing on the prey species of the target stock and the predators of the prey species, which can affect the growth and recruitment of the target stock.
Habitat degradation: Habitat degradation can affect the growth, reproduction, and survival of fish populations, so assessments should consider the effects of human activities such as pollution and habitat destruction on the target stock and its habitat.
Non-target species: By considering the bycatch and discards of non-target species, the assessment can take into account the effects of fishing on these species and their role in the ecosystem.
More to a fish than just the fillets
Fish populations provide a variety of ecosystem services, including:
Food provision: an important source of protein for humans and other animals, and support commercial and subsistence fisheries globally.
Biodiversity: support the diversity and abundance of other species in the ecosystem, such as by providing food for predators or by maintaining the structure of aquatic habitats.
Climate regulation: help regulate the Earth’s climate by removing carbon from the atmosphere and storing it in their bodies and in the ocean floor when they die.
Water purification: help purify water by consuming and breaking down pollutants, such as excess nutrients from agricultural runoff.
Flood control: help prevent flooding by maintaining the health of wetlands and other aquatic habitats that can absorb and store water.
Cultural services: provide cultural and recreational services, such as supporting traditional and sport fishing, and are a part of cultural heritage for many communities.
Economic services: provide economic services, such as supporting commercial and subsistence fishing, aquaculture, and ecotourism.
Rebuilding a management framework
Rebuilding times for depleted fish or lobster populations can vary depending on several factors, such as the species, the level of depletion, and the management measures implemented. Generally, rebuilding times can range from several years to several decades. The specific rebuilding time will depend on the species biology, the rate of reproduction, and the mortality rate. The management measures implemented will also play a role in the rebuilding time, such as catch limits, protected areas, and habitat restoration.
The lengthy rebuilding times for fish, shellfish and lobster stocks emphasize the importance of fishing pressure being constrained to the limits informed by stock assessments.
By following stock assessment-informed limits, fishing can be managed at a sustainable level, allowing fish populations to recover and rebuild and ensuring an enjoyable fishing experience for all legitimate extractive users.
